Polyethylene naphthalenedicarboxylate (hereafter, this may be abbreviated as PEN in some case) is superior in basic physical properties such as heat resistance, gas barrier-property, chemical resistance and the like to polyethylene terephthalate. Because of these merits, polyethylene naphthalenedicarboxylate is increasing its uses as a bottle (container), a sheet material and others in recent years. For these uses, many proposals of blending with polyethylene terephthalate have been made beside the single use of polyethylene naphthalenedicarboxylate.
As for a material to be used for a bottle, especially a beverage bottle, it is strongly required that the material has excellent transparency from the view point of commercial value of a bottle itself or a beverage product.
Polyethylene naphthalenedicarboxylate has a lower crystallization speed than polyethylene terephthalate, and the problem of whitening on blow molding is not so extreme as in polyethylene terephthalate. However, when a certain catalyst composition is used, there is observed a problem of the increasing of the haze of a bottle on molding.
As for catalyst used for producing polyethylene naphthalenedicarboxylate, a catalyst same as that used for producing polyethylene terephthalate is generally used.
For example, Japanese Unexamined Patent Publication No. 7-258394 (applied as Patent Application No. 6-50250 on Mar. 22, 1994, and opened in an unexamined publication on Oct. 9, 1995) describes a method for obtaining polyethylene naphthalate having excellent transparency with a specific catalyst composition in the presence of an antimony compound.
Japanese Unexamined Patent Publication No. 8-157582 (applied as Patent Application No. 6-331921 on Dec. 8, 1994, and opened in an unexamined publication on Jun. 18, 1996) describes a method for suppressing the occurrence of fine foreign matters by adding a specific nitrogen-containing basic compound in polyethylene naphthalate which uses an antimony compound as a catalyst.
Further, Japanese Unexamined Patent Publication No. 10-245433 (applied as Patent Application No. 9-51636 on Mar. 6, 1997, and opened in an unexamined publication on Sep. 14, 1998) describes a method in which polyethylene naphthalate is obtained by adding trimethyl phosphate as a phosphorous compound in the presence of an antimony compound/a cobalt compound.
The inventors of the present invention have found that it is effective to add a specific phosphorous compound for obtaining polyethylene naphthalate having excellent transparency and hue in the coexistence of an antimony compound/a cobalt compound or under conditions where nitrogen-containing basic compound is absent, and they have reached the present invention.